A brazing method has been widely used as a method for joining parts having a large number of small joints (e.g., aluminum heat exchanger and machine parts). When brazing aluminum (including an aluminum alloy), it is indispensable to break the oxide film that covers the surface of the material so that the molten filler metal comes in contact with the matrix or another molten filler metal. The oxide film can be broken by utilizing a method that utilizes flux, or a vacuum heating method, for example. These methods have been put to practical use.
The brazing method has been applied to various fields. The brazing method has been most typically applied to automotive heat exchangers. Most of the automotive heat exchangers (e.g., radiator, heater, condenser, and evaporator) are made of aluminum, and produced by applying the brazing method. A method that applies a non-corrosive flux to the material, followed by heating in nitrogen gas is most widely used at present.
In recent years, a heat exchanger provided with electronic parts (e.g., inverter cooler) has been used along with a change in driveline (e.g., electric car and hybrid car), and a flux residue has posed problems. Therefore, some of the inverter coolers are produced using a vacuum brazing method that does not utilize a flux. However, since the vacuum brazing method utilizes a heating furnace, that increases the equipment and maintenance costs, and has problems as to productivity and brazing stability, a brazing method that is implemented in a nitrogen gas furnace without using a flux has been increasingly desired.
For example, a method that utilizes a brazing sheet that is clad with a filler metal to which a small amount of Be is added, and performs an etching treatment in an acid or an alkali, followed by heating for brazing was put to practical use. However, the application range of this method was limited, and this method is rarely used at present. The application range of this method was limited for the following reasons.    (1) Since the brazability is poor as compared with the method that applies flux, brazing failure easily occurs.    (2) A toxic element (i.e., Be) is included in the filler metal even in a small amount.
The problem (1) is a fundamental problem. For example, even if easy brazing (e.g., brazing a fin and a tube) can be performed without a problem, it may be difficult to reliably braze an area where it is necessary to prevent leakage (e.g., an area in which a tube is inserted into a header, or a joint at the outer circumference of a hollow heat exchanger formed using a pressed sheet). Therefore, the application range is limited to a heat exchanger that can be easily brazed (e.g., a stacked-type heat exchanger that is mainly brazed in a plane, or a heat sink that is produced by brazing a fin to a base such as an extruded shape). The problem (2) (i.e., the toxicity of Be) is serious in the fields of food, medical equipment, and automotive heat exchangers, and the above method may generally be rejected for this reason.
A method has been proposed that brazes aluminum members in a non-oxidizing atmosphere without using a flux in a state in which an Al—Si-based filler metal that includes Na, K, or Li is positioned between the aluminum members. However, this method has a problem in that sufficient brazability cannot necessarily be obtained.